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[UPDATED, 4/30/2010] The oil slick in the Gulf of Mexico reportedly reached shore early Friday morning, and wildlife specialists are preparing for what the government is calling “a spill of national significance” that may reach the fragile Louisiana coast as early as Thursday night.

Some 5,000 barrels a day, or 200,000 gallons (more than five times more than earlier estimates) are pouring into the ocean from a broken pipe 5,000 feet below the surface following the Deepwater Horizon rig incident. So far, efforts to stem the flow have failed, and controlled burns haven’t eradicated the slick. Wildlife officials are preparing for what may be the worst oil spill in history (Department of Homeland Security Secretary Janet Napolitano, Interior Department Secretary Ken Salazar, and EPA Administrator Lisa Jackson are heading to the Gulf Coast on Friday.)

"The terrible loss of 11 workers may be just the beginning of this tragedy as the oil slick spreads toward sensitive coastal areas vital to birds and marine life and to all the communities that depend on them," said Melanie Driscoll, an Audubon bird conservation director, who is monitoring the situation from her base in Louisiana. "For birds, the timing could not be worse; they are breeding, nesting and especially vulnerable in many of the places where the oil could come ashore."

“As the encroachment of oil into coastal zones appears imminent, primary concerns include potential impacts to 20 coastal national wildlife refuges within the possible trajectory of the spill,” said Tom MacKenzie, a public affairs specialist for the U.S. Fish and Wildlife Service. Cleanup teams are putting in place booms to try and protect the areas most at risk, including Breton National Wildlife Refuge. “[Breton] Refuge staff have estimated more than 34,000 birds, including 2,000 pairs of pelicans, 5,000 pairs of royal terns, 5,000 pairs of caspian terns and 5,000 pairs of feeding, loafing and nesting gulls and other shore birds,” says MacKenzie.

Here’s a look at some of the birds and other wildlife experts are most concerned about:

Brown Pelican: Louisiana's state bird nests on barrier islands and feeds near shore. Their breeding season just began and many pairs are already incubating eggs. Removed from the Endangered Species list only last November, brown pelicans remain vulnerable to storms, habitat loss and other pressures. Their relatively low reproductive rate means any disruption to their breeding cycle could have serious effects on the population. [Audubon] Photo: Gary M. Stolz, FWS

Reddish Egret: Populations of these large, strictly coastal egrets have dwindled due to habitat loss and disturbance. As specialized residents of coastal environments, they have nowhere else to go if their feeding and nesting grounds are fouled by oil. [Audubon] Photo: James C. Leupold, FWS

Roseate Spoonbill: Like many herons and egrets, the roseate spoonbill feeds in marshes and along the coast and nests in large colonies that will be vulnerable if oil comes ashore. [Audubon] Photo: NASA

Black Skimmer: Along with other beach-nesting terns and gulls, like the least tern and royal tern, black skimmers nest and roost in groups on barrier islands and beaches. They feed on fish and other marine life. Roosting and nesting on the sand and plunging into the water to fish, they are extremely vulnerable oil on the surface or washing ashore. [Audubon] Photo: Gary Kramer, FWS

Snowy Plover: This bird and other shorebirds nest on the ground on barrier islands and beaches. They feed on small invertebrates along the beach or, in the case of oystercatchers, on oysters. They’re at risk if oil comes ashore or affects their food sources. [Audubon] Photo: BLM

Mottled Duck: Many marsh-dwelling birds are extremely secretive, hindering understanding of their population dynamics. Recovery efforts would be difficult or impossible if oil accumulates in the coastal salt marshes where they live. [Audubon] Photo: Peter Wallack, Wikimedia Commons

Sea turtles: Several species of threatened sea turtles live, breed, and migrate in the Gulf. Although surprisingly robust when faced with physical damage such as shark attacks or boat strikes, these creatures are highly sensitive to oil. Oil causes increased egg mortality and developmental defects, direct mortality due to oiling in hatchlings, juveniles, and adults; and negative impacts to the skin, blood, digestive and immune systems, and salt glands. [NOAA]Photo: NOAA

Blue fin tuna: The spill is near the fish’s spawning grounds, which they inhabit from mid-April to June each year. The highly migratory tuna’s numbers are already quite low, in large part due to overfishing. [NOAA] Photo: Wikimedia Commons

Whale sharks: These polka-dotted fish can grow to be 40 feet long and sometimes gather in large numbers to feast on seasonal plankton blooms, as happened last summer when more than 400 appeared in the northern Gulf, baffling scientists. [Gulf Restoration Network] Photo: Zac Wolf, Wikimedia Commons (Georgia Aquarium)

Sperm whales: As of Wednesday, NOAA had received reports of at least seven sperm whales seen in the oil-impacted area. These mammals are endangered. [NOAA, NOLA] Photo: NMFS Northeast Fisheries Science Center

Oysters: The spill is nearing the rich oyster beds at the mouth of the Mississippi River. "When the oil starts to settle, it'll smother the oyster beds. It'll kill the oysters," said Gregory Bossart, chief veterinary officer for the Georgia Aquarium in Atlanta. Bossart points out that oil smothers plankton, which are fed on by crabs, mussels, oysters, and shrimp. [CNN]